Mobile devices such as tablets and smartphones typically use lithium batteries that produce relatively high voltages such as 4 V. Although such batteries are efficient and have relatively long lifespans, their resulting voltages are too high for modern transistor processing nodes. For example, it is conventional for digital cores to operate with power supply voltages of less than 1 V. Mobile devices thus typically include a buck converter for regulating the relatively-high battery voltage into a relatively-low power supply voltage for its digital circuits.
Buck converters (and other types of switching power converters) will typically include a bias current generator to generate a bias current (for example, 10 μA) that is used by various other converter components. An example conventional bias current generator 100 is shown in FIG. 1. An operational amplifier 105 drives a gate of an NMOS output transistor M1 with a control voltage responsive to the difference between a source voltage for transistor M1 and a reference voltage such as a bandgap reference voltage (VBG). The source of transistor M1 couples to ground through a resistance R1. Feedback through operational amp 105 will thus keep the source voltage for transistor M1 equal to the reference voltage VGB such that a bias current I conducted through resistor R1 equals VBG/R1 by Ohm's law. This bias current I is then mirrored through current mirrors formed by a diode-connected PMOS transistor P1 having its gate (and drain) voltage also driving the gates of PMOS transistors P2 and P3 that have their sources coupled to a power supply voltage VDD in common with the source of diode-connected transistor P1. Transistors P2 and P3 will thus mirror the bias current I into a secondary current that is proportional to the bias current with the proportionality depending upon the size ratios between transistors P1, P2, and P3.
Note that it is common for a mobile device processor to enter a sleep mode during periods of inactivity to conserve battery power. It is desirable for the mobile device processor to be able to wake up and enter normal operation from such a sleep mode as quickly as possible. To enable such a fast transition, bias current generator 100 may be left running during the sleep mode (which may also be denoted as a dormant mode or an inactive period) but such constant operation lowers efficiency due to the resulting drain of the bias and secondary currents. But switching off bias current generator 100 is also problematic since feedback through operational amplifier 105 takes some time to settle such that stable bias current generation typically requires several micro-seconds, which lowers the transition speed from sleep mode to normal operation.
Accordingly, there is a need in the art for improved bias current generators that may be powered off during inactive periods but which can quickly settle after being powered on during active periods to provide a stable bias current with minimal delay.